The invention relates generally to antennas for use in communication and navigation systems. In particular, the invention relates to a single or dual band antenna array for receiving and/or transmitting circularly polarized signals in the presence of hostile or unintentional interference.
Many contemporary communications and navigation products have been developed that rely on earth-orbiting satellites to provide necessary communications and navigation signals. Examples of such products include satellite navigation systems, satellite tracking and locator systems (e.g., GPS), and communications systems (e.g., NAVSTAR) that rely on satellites to relay the communications signals from one station to another. In order for these products to be operationally useful as hand-held equipment, the antennas they employ should be small (comparable or smaller in size than the receiver itself).
Several types of antennas are now used with hand-held GPS receivers. All are relatively compact and can receive circularly polarized signals from any direction above the ground (e.g., hemispherical coverage, although gain along the ground or horizon can be reduced). The requirement for compact size has several performance benefits in addition to its obvious portability. It enables the radiation pattern of an antenna to have slowly varying gain and low frequency dispersion over most of the field of view. The latter is important to provide the desired location accuracy. But any communications or navigation system is susceptible to degradation due to interfering conditions. Carrier signals are vulnerable to interruption by natural phenomena, interference from other signals or countermeasures. Countermeasures may take the form of a variety of jamming schemes whose sole purpose is to disrupt the operation of a receiver.
Multipath is a significant problem in both navigational and communications systems. It degrades navigational accuracy in GPS systems and can be a source of interference in communications systems. Multipath can be caused by xe2x80x9cstructuralxe2x80x9d reflections from specular reflecting surfaces of numerous scattering sources common to an urban environment such as buildings, large vehicles, aircraft or ships. Alternatively, multipath can be caused by ground reflections at low grazing angles off the moist ground, rooftops, sea surface or a large body of water close to the antenna. Since GPS satellites transmit right-handed circularly polarized (RHCP) signals, and the polarization of a multipath signal after reflection is normally reversed, the rejection of the cross-polarized (left-handed circularly polarized, LHCP) signals is important in avoiding multipath problems.
Various types of antennas have been proposed for decreasing the effects of interference. In addition to their large size, most have large numbers of radiating elements that make them unsuitable for use in hand-held devices such as GPS receivers. Additionally, most do not allow operation in more than one frequency band. For the next generation of GPS hand-held receivers being developed for the military, it will be necessary to receive circularly polarized signals in two frequency bands, (L1 and L2). In addition, it is desired to be able to suppress active interference signals (e.g., jammers) by predictively or adaptively placing a radiation pattern minimum in the direction of that interference. This requires one or more auxiliary antennas whose output would be combined with the output of the primary antenna(s) with an adaptive weight to appropriately shape the pattern.
Thus, a need exists for a simple predictive or adaptive antenna array compact enough to be suitable for use in a hand-held device, yet which is capable of receiving and/or transmitting circularly polarized signals in more than one (preferably satellite) frequency band while providing a relatively high gain quasi-hemispherical radiation pattern over those bands.
The present invention provides an antenna array that is operationally useful in hand-held communications and navigation (e.g., GPS) transceivers and receivers. Some embodiments of the antenna array are capable of receiving and/or transmitting circularly polarized signals within separate frequency bands. In addition, in order to suppress incident signals that would potentially interfere with the desire signal, a pattern minimum (predictive or adaptive) can be steered in the direction of an interfering signal. An appropriate amplitude and phase weight is applied to the output of an auxiliary antenna such that, when combined with the output of a primary antenna, generates the pattern minimum. The pattern minimum created by this two-element antenna array will have a conical shape with the axis of the cone along a line separating the primary and auxiliary antenna elements. Because most interfering signals are anticipated to arrive from a direction close to the horizon, a vertical displacement between the primary and auxiliary elements is preferred, though not required, in order to steer the pattern minimum along the horizon while providing maximum pattern gain in the upper hemisphere.
Antennas that are electrically small, passive and have low ohmic loss will have a relatively narrow bandwidth and high Q. Essentially such antennas are resonant circuits, with the non-ohmic loss representing radiation. By modifying the design of such antennas to be the equivalent of a double-tuned resonant circuit, they may operated efficiently in two distinct frequency bands (e.g., L1 and L2). This is a technique used in certain embodiments of the present invention, wherein the technique is applied to both a quadrifilar helix antenna (QHA) and a microstrip patch antenna (patch).
In a first embodiment, the present invention provides a spatial null steering antenna array comprised of a primary QHA resonating circularly polarized radiation in a desired frequency band and means for feeding the QHA in phase quadrature (i.e., feed signals having relative phase differences of 0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0 from an input signal) such as described below. Coaxially aligned and vertically displaced from the QHA is an auxiliary patch antenna resonant in the same frequency band. The patch is stacked atop a dielectric substrate layer and a ground plate. The positions of the patch, substrate layer and patch ground plate are fixed relative to the QHA. A means for applying an appropriate amplitude and phase weight to feed signals feeding the patch results in a combined circularly polarized radiation pattern minimum at selected elevation angles. The desired frequency band is preferably a satellite frequency band, such as either of the L1 or L2 bands.
Several QHA designs are known to artisans. In one bottom-fed embodiment, the QHA is comprised of four radiating elements arranged helically with a left-hand twist to define a cylinder of constant radius, each radiating element having an electrically open end below the patch ground plate and another end having an electrically conductive feed location, and a QHA ground plane coaxially positioned below the four radiating elements. In some embodiments, the radiating elements have sufficient rigidity to be self-support, while in others they are structurally supported. In other top-fed embodiments, the need for a QHA ground plane is eliminated by taking advantage of the inherent backfire nature of QHA""s. In these top-fed embodiments, the radiating elements have a right-hand twist and the electrically-open and feed-location ends are inverted. The radiating elements may be comprised of strips of electrically conductive material printed on a dielectric support. The radiating elements each has a length preferably less than or equal to the wavelength in free space of the signals being fed, and each preferably completes one helical turn about the (imaginary) defined cylinder.
In certain preferred embodiments, the auxiliary patch feed signals are transported from the patch feed means to the patch via a conduit located coaxially within and extending above the QHA. The conduit also preferably transports the QHA feed signals in certain top-fed embodiments. In some embodiments, the conduit also serves as portion of a support mechanism for fixing the position of the patch, substrate layer and patch ground plate relative to the QHA. The patch may be fed predictively, as mentioned above, with appropriately amplitude and phase weighted feed signals to achieve a pattern minimum at the horizon. Alternatively, the patch may be fed by an adaptive weighting feed network in order to steer the pattern minimum to other elevations from where interfering signals may be arriving.
In another embodiment, the present invention provides a dual-band spatial null steering antenna array comprised of a primary dual-band QHA and a coaxially aligned and vertically displaced auxiliary stacked patch antenna, each of which is resonant in a first frequency band and a second frequency band, means for feeding the dual-band QHA to produce circularly polarized radiation in the first frequency band and the second frequency band, and means for feeding the stacked patch antenna to produced circularly polarized radiation such that, when combined with the output of the dual-band QHA, generates pattern minima in the first and second frequency bands at selected elevation angles. The first and second frequency bands are preferably, but not limited to, the L2 and L1 GPS bands, respectively.
An auxiliary stacked patch antenna as used herein comprises a patch ground plane coaxially aligned with and vertically displaced above the dual-band quadrifilar helix antenna, and a first auxiliary patch resonant in the first frequency band and a second auxiliary patch resonant in the second frequency band coaxially aligned and vertically stacked on a dielectric substrate layer on top of the patch ground plate. The first auxiliary patch and second auxiliary patch may be feed predictively or adaptively by the patch feed network in each of the two frequency bands. Appropriate feeding may occur simultaneously in both frequency bands, or alternating between the two frequency bands as desired.
As in the single frequency band embodiments of the present invention, a means for supporting the position of the auxiliary stacked patch antenna relative to the dual-band QHA is preferably employed, as well as an optional means for supporting the structure of the dual-band QHA. The auxiliary stacked patch support means may similarly comprise a conduit mechanism through which patch feed signals are transported from the patch feed network.
Several dual-band QHA""s are known and may be employed, but in preferred embodiments the present invention employs trap-loaded QHA""s as described below. Generally speaking, trap-loaded QHA""s employ radiating elements in which are inserted parallel LC circuits whose impedance in the second frequency band is significantly higher than in the first frequency band, thus effecting the double tuning mentioned above. The details of the trap-loaded QHA and the combination of its output radiation pattern with that of the auxiliary stacked patch antenna will be described in detail below.